1. Field of the Invention
The present invention relates to vehicle surroundings monitoring apparatuses, and more particularly, to a vehicle surroundings monitoring apparatus capable of controlling the exposure amount of a camera that takes an image of the surroundings of a vehicle in which the apparatus is mounted.
2. Description of the Related Art
In recent years, techniques of detecting objects existing around a vehicle, such as a car, by analyzing an image taken by an image pickup means, such as a charge coupled device (CCD) camera, have been developed (for example, see Japanese Unexamined Patent Application Publication No. 7-225892). These technologies are applied to safe driving technologies for vehicles. For safe driving, for example, the probability of a collision with a detected object is judged, an alarm is given to alert the driver, and automatic steering and automatic brake control are exerted in order to avoid the collision.
In this object detection, it is important to accurately detect obstacles, such as a preceding vehicle, an oncoming vehicle, a pedestrian on the road, and a car parked at the roadside. In order to avoid a collision with a detected obstacle, collision avoidance control is performed. For example, even when the preceding vehicle stops at stop signals, it is not an obstacle, and a driver's own vehicle needs to properly stop behind the preceding vehicle without passing by the preceding vehicle. Since control different from control for other obstacles is required with respect to the preceding vehicle in this way, it is particularly necessary to accurately detect the preceding vehicle.
In order to accurately detect these objects, particularly the preceding vehicle, from an image taken by the image pickup means, the image must be taken by properly performing exposure control in the camera. The image pickup means, such as a CCD camera, normally has this function of automatically performing proper exposure control.
Unfortunately, when the image pickup means takes an image of an environment in which a bright area and a dark area are mixed, for example, when the preceding vehicle enters a tunnel or comes out of the tunnel, that is, when the vehicle having the image pickup means is moving in a bright environment before the tunnel and the inside of the tunnel is dark, the shutter speed is shortened or an iris diaphragm is narrowed in accordance with the bright area during normal automatic exposure control of the image pickup means. For this reason, a taken image of the inside of the tunnel is very dark in general.
When the preceding vehicle enters a dark area, such as the inside of the tunnel, in this state, the preceding vehicle itself becomes dark, and it is difficult to distinguish between the dark surroundings and the dark preceding vehicle. In this case, detection of the preceding vehicle is sometimes impossible. In order to solve this problem, for example, Japanese Unexamined Patent Application Publication No. 7-81459 proposes a car driving control apparatus that adjusts an iris diaphragm of the image pickup means by detecting the brightness of the preceding vehicle itself.
Japanese Unexamined Patent Application Publication No. 11-139225 proposes a vehicle control apparatus that keeps a vehicle in a state suitable for driving in a tunnel. In this apparatus, when the entrance of a tunnel is detected, an air circulation path of an air conditioner in the vehicle is switched to internal recirculation, and the exposure amount of the image pickup means is increased. Japanese Unexamined Patent Application Publication No. 11-205663 proposes an exposure control apparatus that controls the exposure amount of the image pickup means so as to detect a traffic line when a tunnel is detected. In the present invention, continuous lines and broken lines marked on the road surface, for example, a road center line such as a no-passing line, a traffic-lane boundary line, and a separating line for separating a side strip and a roadway, are referred to as traffic lines.
In the apparatus disclosed in Japanese Unexamined Patent Application Publication No. 7-81459, when the preceding vehicle moving in a bright place becomes dark, for example, by happening to enter the shade of a tree, iris control of the image pickup means is performed. However, even if the preceding vehicle itself becomes dark, it can be detected as long as it is bright in front of the preceding vehicle and a boundary between the preceding vehicle and the surroundings can be clearly found on an image taken by the image pickup means. Therefore, in this case, iris control of the image pickup means is unnecessary from the standpoint of accurate detection of the preceding vehicle. It is rather preferable that iris control not be performed in this case, because the control may have an influence on detection of other objects such as traffic lines.
In the apparatus disclosed in Japanese Unexamined Patent Application Publication No. 11-139225, the exposure amount of the image pickup means is controlled as normal automatic exposure control when the vehicle enters the tunnel. Since exposure control is not performed while the vehicle is moving in a bright environment, it is difficult to take an accurate image of the preceding vehicle moving in a dark environment so that the image is distinct from the dark surroundings.
In the apparatus disclosed in Japanese Unexamined Patent Application Publication No. 11-205663, in order to detect a traffic line in both a bright environment before a detected tunnel and a dark environment in the tunnel, the exposure amount of the image pickup means is increased to an upper limit such that the brightness of the traffic line does not become saturated in the bright environment before the tunnel and such that the traffic line can be distinguished from a road surface. However, this exposure amount control is performed strictly for the purpose of detecting the traffic line. It is not certain that this exposure amount control can allow an image of the preceding vehicle in the dark environment in the tunnel to be accurately taken so as to be distinct from the dark surroundings.
Forced exposure control needs to be performed so as to accurately detect the preceding vehicle. However, if forced control is performed independently of normal automatic exposure control by the image pickup means, erroneous detection of objects different from the preceding vehicle may be caused. Therefore, it is preferable to perform automatic control so that forced exposure control is not performed in an unnecessary condition.